1. Field of the Invention
The present invention is directed to a method for limiting residual echo in a speech signal-carrying channel or line, as well as to an apparatus for implementing the method.
2. Description of the Prior Art
Generally, echo cancelling circuits for speech signals contain a residual echo limiter, designated with the abbreviation NLP (non-linear process). This circuit part is activated only when there is no duplex speech, and thus only the disturbing residual echo signal is transmitted to the speaking subscriber at the remote end of the channel or line. If the proximate-end subscriber is speaking, or both subscribers are speaking at once, the NLP is suppressed. In contrast, an activated NLP blocks the echo, which cannot be further compensated, and which is caused by non-linearities in the echo path, or also by PCM coding.
A method of this type is discussed in German OS 41 35 790. A control stage connects signals through to the transmission output if at that time they are still above the level of the residual echo signal. The residual echo limiter is in effect only when the remote-end subscriber is also speaking, but is never in effect during duplex speech. In order to recognize with maximum precision the times in which only the remote-end subscriber is speaking, monitoring of the incoming and outgoing signals by means of level and attenuation measurements is required. Since speech comprises considerable fluctuations in loudness level, the measurement means must contain integration elements. With increasing integration time, the precision of the measurement also increases. On the other hand, the measurement values are supposed to rapidly follow change in the echo path. A compromise is thus required.